Glass sheets are quenched to provide tempering or heat strengthening in order to increase the mechanical strength of the glass and hence provide an increased resistance to breakage as compared to annealed glass. In tempering, quenching gas is impinged with the opposite surfaces of the glass sheet to provide rapid cooling thereof such that the finally cooled glass sheet has compressive forces at its surfaces and tensile forces at its center and is thereby stronger and more resistant to breakage. Also, tempered glass has the characteristic of breaking into small dull pieces as compared to the sharp slivers that result when annealed glass is broken. With heat strengthening, quenching gas is also impinged with the opposite surfaces of the glass sheet, but at a much lower rate and thereby provides the surfaces of the glass with compressive forces but at a much lower level than is involved with tempering. Both tempering and heat strengthening can be performed on flat glass sheets such as are conventionally used for architectural purposes and on bent glass sheets such as are conventionally used for vehicle windows.
Glass sheet quenches conventionally include opposed blastheads each of which has elongated plenum housings that are spaced from each other and supply pressurized quenching gas to a heated glass sheet positioned between the blastheads. The plenums are spaced from each other to leave room for spent quenching gas to flow away from the glass sheet and between the plenums for flow out from between the blastheads. Normally the quenching gas is supplied in discrete jets that are spaced along the length of each plenum housing as disclosed by U.S. Pat. No. 3,936,291. Such discrete jets are preferable to an elongated slit through which the quenching gas is supplied as disclosed by U.S. Pat. No. 2,948,990 wherein deflectors are also provided for controlling the direction of gas supplied through such slits. Normally quenching jets are supplied in a parallel relationship to each other to provide tempering of flat glass sheets which are positioned between the opposed blastheads of the quench extending in a perpendicular relationship to the quenching jets. Likewise, quenching of bent glass sheets can also be performed with the quenching jets arranged in a parallel relationship to each other or the quenching jets can be arranged in a generally perpendicular relationship to the bent glass sheets as disclosed by U.S. Pat. No. 3,939,062.
One problem that results in attempting to provide a greater degree of cooling to glass sheets is that a pressure buildup results from the spent quenching gas which cannot escape from between the blastheads through which the quenching gas is supplied. This is a particular problem with quenching of flat glass sheets on roller hearth conveyors as disclosed by U.S. Pat. Nos. 3,806,312, 3,934,970, 3,947,242, and 3,994,711, since the lower side of the glass sheet being quenched is obstructed by conveyor rolls which reduce the area for gas to escape and thereby generate a greater pressure buildup of the spent quenching gas. However, this pressure buildup can also be a problem with quenching of bent glass sheets on an open center mold ring in the manner disclosed in U.S. Pat. No. 4,282,026. In quenching both flat and bent glass sheets, a greater degree of quenching gas must be supplied as the glass sheets become thinner such that the buildup of spent quenching gas is a greater problem than is the case with thicker glass sheets.
In quenching glass sheets, it is important to maintain uniform gas flow and pressures so as to prevent any significant variation in pressure over the surface of the glass sheet being quenched or any variation from one sheet to the next sheet. It is also desirable for quenching gas to flow as efficiently as possible through the blastheads to the glass sheets in order to reduce the energy input necessary to perform the quenching.